DUB: Sometimes generates .di file and sometimes do not.

[BoQsc]

Update:

To allow only `.di` (D Header) output:

Instead of configuration, it would be more correct to make a new 
`build type`.

```
buildType "headerFileOnly" {
	extraDependencyFiles "$PACKAGE_DIR/builds/library.di"
	dflags "-Hf=$PACKAGE_DIR/builds/library.di"
	dflags "-o-"
	SourceFile "library.d"
}
```

Usage:
```
dub --build=headerFileOnly
```

This will produce only the `.di` D header file without `.lib` 
file.